This invention relates to a heating apparatus particularly useful in lining tubular structures such as pipe, particularly, but not exclusively, long lengths of pipe in situ such as underground gas or water and sewer mains.
Metal pipes may be lined with thermoplastic polymers to convey corrosive material such as acids, or the lining may be used simply to rehabilitate or prolong the life of old, existing water or gas supply pipes or other pipes which may carry relatively non-corrosive fluids.
It has been proposed in U.S. Pat. No. 3,429,954 to line metal pipe by inserting into a pipe a longitudinally oriented heat shrinkable, irradiated thermoplastic polymer tubing, the tubing having in its unoriented state an outer diameter greater than the internal diameter of the metal pipe but slightly smaller than the internal diameter when oriented, and heating the tubing to a temperature above the crystalline melting point of the polymer to longitudinally shrink the polymer tubing and cause it to expand radially and fit tightly against the internal wall of the pipe.
The polymer tubings employed in U.S. Pat. No. 3,429,954 were irradiated to cross-link the polymer. This, however, precludes in most cases the use of such pipe from lining long continuous lengths of underground metal pipe where sections of polymer lines must be joined by butt fusion or butt solvent welding to make up the entire length. Cross-linked polymers cannot with confidence be joined by fusion or solvent welding and are inferior in this respect to non-crosslinked thermoplastic polymers.
In the U.S. patent it is also suggested that the same polymer liners be expanded by heating using one of three methods.
(a) application of hot air internally
(b) passing the polymer lined pipe through a furnace so as to apply heat externally
(c) immersing the polymer lined pipe in a propylene glycol bath.
While methods (b) and (c) for obvious reasons are impractical for the relining of long, continuous lengths of underground pipe, method (a) is not effective either because in long pipe sections the air or any other gas or fluid will cool rapidly as it progresses along the pipe and there will be insufficient heat to raise the entire length of the polymer liners to the necessary temperatures.
It is the object of this invention to provide a heating apparatus particularly useful for heating the lining of long lengths of underground pipe that will overcome all of the above mentioned difficulties and problems.